How to add image on 2 materials?
Your question is not clear, you're talking about putting your ice-cream picture in-between the 2 parts but in your screenshot it is on the blue part only. However, what would work in both cases is giving each part its own material, as it is already the case, but reunite the two unwrapped parts, as I've done, and in both the 2 materials, use the same UV (which will be the case by default if you just plug the Texture Coordinate / UV output), then use the picture as an overlay like you've already done for one (with the alpha in the factor). With this nodes organization you will control the position of your ice-cream picture just by moving the UV position in the UV/Image Editor, whether you want it on one part only or in-between. As the UV is common to both materials, what you decide to do with the UV map will impact both the 2 materials.